The use of pavement markings (e.g., paints, tapes, and individually mounted articles) to guide and direct motorists traveling along a roadway is well known. During the daytime the markings may be sufficiently visible under ambient light to effectively signal and guide a motorist. At night, however, especially when the primary source of illumination is the motorist's vehicle headlights, the markings are generally insufficient to adequately guide a motorist because the light from the headlight hits the pavement and marking at a very low angle of incidence and is largely reflected away from the motorist. For this reason, improved pavement markings with retroreflective properties have been employed.
Retroreflection describes the mechanism where light incident on a surface is reflected so that much of the incident beam is directed back towards its source. The most common retroreflective pavement markings, such as lane lines on roadways, are made by dropping transparent glass or ceramic optical elements onto a freshly painted line such that the optical elements become partially embedded therein. The transparent optical elements each act as a spherical lens and thus, the incident light passes through the optical elements to the base paint or sheet striking pigment particles therein. The pigment particles scatter the light redirecting a portion of the light back into the optical element such that a portion is then redirected back towards the light source.
In addition to providing the desired optical effects, pavement markings must withstand road traffic and weathering, adverse weather conditions, and cost constraints.
Somewhat vertical or upwardly disposed surfaces provide better orientation for retroreflection than do horizontal surfaces; therefore, numerous attempts have been made to incorporate vertical surfaces in pavement markings, typically by providing protrusions in the marking surface. In addition, vertical surfaces may prevent the build-up of a layer of water over the retroreflective surface during rainy weather which otherwise interferes with the retroreflection mechanism.
One means of providing vertical surfaces is to place raised pavement markers at intervals along a pavement marking line (e.g., U.S. Pat. Nos. 3,292,507; 4,875,798). These markers are relatively large, generally several centimeters in width and 5 to 20 millimeters in height. Typically, the markers require assembling together different components, some of which were previously individually molded or casted. Therefore, the markers are relatively expensive to manufacture. The size of the markers subjects them to substantial impact forces from passing vehicles. As a result, the markers must be substantially secured to the pavement, increasing the installation costs and removal costs when they wear out. Moreover, because the markers are applied at intervals, the bright spots of light are discontinuous, rather than the desired continuous bright line.
Embossed pavement marking tapes are a second means of providing vertical surfaces (e.g., U.S. Pat. Nos. 4,388,359, 4,069,281, and 5,417,515). Selective placement of transparent optical elements on the vertical sides of the embossed protrusions results in a highly effective marking material. However, such tapes are relatively expensive compared to conventional painted markings, and thus their use is often limited to critical areas such as unlighted intersections and railway crossings. Also, these embossed tapes are constructed with polymeric materials which are susceptible to wear.
A third means of providing vertical surfaces for retroreflection is a composite retroreflective element or aggregate (e.g., U.S. Pat. Nos. 3,254,563, 4,983,458). Many variations are known, but the retroreflective elements essentially have a core with optical elements embedded in the core surface. Some known embodiments also contain optical elements dispersed throughout the core that become exposed upon wear. The core may be irregular in shape or may be shaped into spheres, tetrahedrons, discs, square tiles, etc. Retroreflective elements are advantageous because they can be embedded into inexpensive painted markings.
Retroreflective elements are largely comprised of polymeric cores or binders. A pigmented core or binder often serves as a diffuse reflector. This arrangement allows spherical optical elements to be used on either horizontal or vertical surfaces. Other constructions have transparent optical elements comprising a specular reflector such as metallic silver. The metallic surface directs light back towards the source and a pigmented core is not necessary. Because of the geometry of the optics, a specular coated optical element would not be as effective if embedded in a pavement marking paint (a horizontal surface), and would be more highly effective if embedded in the vertical surfaces of a retroreflective element.
Another retroreflective element construction, U.S. Pat. No. 3,252,376, only has silvered glass flakes serving as a specular reflector on the surface of a spherical polymeric core without the use of spherical optical elements.
Another known construction is a retroreflective element where a plastic globule (lens) refracts incident light onto a layer of glass optical elements attached to the bottom portion of the globule. The glass optical elements then focus the light onto a specular coating or film located below the optical elements, where the light is then reflected back along the original path towards the source (e.g., U.S. Pat. Nos. 4,072,403; 4,652,172; 5,268,789).
Shaped polymeric retroreflective elements with a pigmented core and glass optical elements embedded in the vertical surfaces are disclosed in U.S. Pat. No. 3,418,896. These retroreflective elements are formed by extruding the pigmented polymer into rods of different cross-sectional shape. Glass optical elements are embedded into the surface of the polymer before it hardens, then the rods are sliced to form the desired elements.
Polymeric retroreflective elements are undesirably susceptible to wear, especially in high traffic regions, and to degradation due to weathering. In an attempt to overcome these limitations, retroreflective elements were constructed having a ceramic core and glass optical elements with a metallic specular coating.
One type of construction is a rock or glass sphere core (U.S. Pat. Nos. 3,043,196 and 3,175,935) covered by a polymeric binder with glass optical elements having a specular metallic coating embedded in the polymeric coating.
Another construction disclosed in U.S. Pat. No. 3,556,637 has a glass sphere and a layer of glass optical elements attached to the bottom of the glass sphere with a polymeric binder. A metallic film below the glass optical elements acts as a specular reflector.
Other known constructions include a composite lens element serving both as a retroreflective element and a skid-resistant particle (EP 0,322,671). The skid-resistant particle which acts as a core (either a corundum particle or glass sphere) is coated with a pigmented polymeric binder which acts as a diffuse reflector.
A ceramic element having glass optical elements embedded throughout a glass core and at the surface of the core is disclosed in U.S. Pat. No. 3,171,827. A thin metallic film separates the optical elements and the glass core to provide an efficient specular retroreflective system. Alternatively, optical elements having a high refractive index (greater than 2.0) are used. These high refractive index optical elements are said to be capable or reflecting light without the need for a reflective backing.
A ceramic retroreflective element having a transparent glass sphere with smaller glass optical elements embedded into the surface is disclosed in U.S. Pat. Nos. 3,274,888 and 3,486,952. Again a thin metallic film separates the optical elements and the glass sphere to provide an efficient specular retroreflective system. The elements are formed by first coating the glass spheres with metallized optical elements using a temporary polymeric binder. The coated spheres are then tumbled with excess optical elements in a rotary kiln. When the temperature exceeds the softening temperature of the glass spheres, the optical elements embed themselves into the surface of the spheres. Later the film is etched away from the exposed portion of the optical elements.
WO 97/28471 discloses a retroreflective element comprising an opacified ceramic core and ceramic optical elements partially embedded into the core. The diffuse reflecting ceramic core, in combination with the transparent optical elements embedded in the surface, provides a surprisingly bright retroreflective element without the gray coloration and the susceptibility to corrosion associated with metallic specular reflectors. Although these all-ceramic retroreflective elements have greatly improved resistance to wear and the effects of weathering, enhanced crush-resistance is desirable to increase the life of the retroreflective element.